The production of polarizing materials, especially plastic materials in sheet form, is well known to the art. The production process generally comprehends two basic steps: first, light-polarizing particles are dispersed in a suspending medium; and, second, the medium is caused to flow under stress, such as by extruding, rolling, or stretching, in order to elongate the particles, where necessary, and to align the particles in the direction of medium flow. Although the vast bulk of the polarizing materials marketed commercially has utilized an organic plastic as the suspending medium with organic and/or inorganic particles and/or molecules being dispersed therewithin, because of the inherent low surface hardness, relatively high moisture susceptibility, low temperature capability, and poor scratch resistance of organic plastics, investigations have been undertaken sporadically to manufacture polarizing glasses. Such research has been most active in the field of ophthalmic applications where high surface hardness and good scratch resistance are important.
Polarizing glasses have been prepared wherein very fine metallic particles and inorganic crystals have comprised polarizing materials. Two methods for making polarizing glasses have been disclosed in the recent patent literature. U.S. Pat. No. 4,125,405 describes the preparation of photochromic polarizing glasses effected through the photolytic reduction of silver and U.S. Pat. No. 3,653,863 discloses the manufacture of polarizing glasses via stretching the glass.
The first method involves subjecting a photochromic glass, wherein a silver halide selected from the group of AgCl, AgBr, and AgI constitutes the photochromic agent, while the glass is in the darkened state to a high intensity source of linearly-polarized visible light, a laser supplying a convenient source of such radiation. The practical aspects of providing such an exposure to the glass have rendered the process intrinsically expensive and slow. Furthermore, non-photochromic silver-containing glasses cannot be made polarizing by that procedure.
The second method is also specifically directed to photochromic glasses wherein a silver halide comprises the photochromic agent. The technique contemplates stretching the photochromic glasses, while at temperatures between the annealing and softening points of the glass, to simultaneously elongate the silver halide particles to an ellipsoidal configuration (conventionally demonstrating a length-to-width ratio, conventionally termed the "aspect ratio", ranging between about 2:1-5:1) and align the elongated particles. The stretched glass is cooled quickly to inhibit the elongated particles from returning to their original shape (respheroidizing).
This stretching technique is subject to several limitations. For example, the redrawing or stretching comprehends placing the glass under high tensile stress and glass is weak in tension. Consequently, although the maximum stress typically obtainable in re-drawing is about 6000 psi, ruptures of the glass at much lower stress levels are not uncommon. Inasmuch as the polarizing character of the stretched glass is dependent upon the maximum stress during re-drawing, premature ruptures not only interrupt the process but create undesirable rejects. Another problem not infrequently encountered in the re-draw process is related to the fact that the stretching is normally conducted at relatively high temperatures, i.e., approximating the softening point of the glass, because lower stresses can be utilized at those temperatures. This action, however, creates a problem with photochromic glass since the photochromic properties are quite sensitive to heat treatment. Haziness and slow fading are two commonly-experienced undesirable characteristics resulting from high temperature heating in the stretching procedure. A further problem witnessed in the re-draw process is the difficulty in controlling the shape and size of the product. The glass to be re-drawn is customarily in the form of small bars. It is well-nigh impossible to generate wide, uniformly-thin sheets of glass, such as would be useful in ophthalmic applications, because of the large reduction in glass body dimensions inherent in the re-draw process. Non-photochromic, silver-containing glasses can be made polarizing by stretching, thereby elongating the silver particles therein, but the same problems are experienced therewith as outlined above.
Extrusion as a forming method has been long recognized in the glass and ceramic arts for producing articles having an axis normal to a fixed cross axis. Extrusion has advantages of speed and ready control of article geometry when compared with a re-draw or stretching process. Accordingly, research has been initiated to develop a method for extruding glass compositions to generate polarizing properties therein.
The shaping of articles through the extrusion of molten glass is known to the arts. The basic element of the process is to force molten glass having a desired viscosity through the orifice of a die. In the more conventional systems, air pressure constitutes the force applied to move the glass through the orifice, although the use of mechanical means for exerting force on the molten glass is discussed in such literature as U.S. Pat. No. 3,467,513. The flow of glass is a function of its viscosity which, in turn, is a function of the temperature of the glass. Consequently, temperature control throughout the extrusion chamber is of vital signficance and particularly is this true in the area of the die aperture. Of somewhat less importance, but still of considerable moment, is regulation of the temperature of the glass subsequent to its leaving the die.
Different means have been proposed for applying heat to a die chamber. For example, U.S. Pat. No. 3,372,445 discloses the use of heating electrodes and U.S. Pat. No. 3,467,513 describes the generation of heat through the use of resistance windings. A rather sophisticated method for controlling the temperature in the die chamber is illustrated in U.S. Pat. No. 3,038,201 where a temperature gradient is produced therein to induce differential heating of the die. The application of fluid coolants to rigidify the extrudate as it leaves the die is noted in U.S. Pat. No. 2,423,260 and U.S. Pat. No. 3,796,532.
As has been explained above, one of the primary problems encountered in the stretching technique for preparing polarizing glasses has been the ready tendency of the silver or silver halide particles to spheroidize at high temperature. This phenomenon occurs because the particles act to minimize surface energy after being elongated. This occurrence is likewise a vital factor which must be faced in extruding glass.